When fibre material is ground in e.g. grinding apparatuses of disc type aqueous steam or other vapour is generated during the grinding operation as a result of the high power imposed on the grinding apparatus and thereby a high steam pressure will arise between the grinding segments of the grinding discs. This causes several inconveniences. The steam pressure produces high axial forces, especially at the outer part of the periphery of the grinding discs, which loads the grinding apparatus structure as deposits etc. and also causes bending of the grinding discs so that the grinding segments lose their parallelism. Another disadvantage is the influence of the steam upon the grinding goods, i.e. the fibre material. Thus in the grinding groove the steam pressure generally follows a curve which increases from the inner periphery of the grinding groove to a pressure centre somewhere on the outer half of the grinding discs in order later to sink again towards the outer periphery of the grinding discs. Part of the steam tends to flow back from this pressure centre to the centre of the grinding discs counter current to the direction of movement of the grinding goods, while another part of the steam from the pressure centre rushes outwards towards the outer periphery of the grinding discs while pulling with it the fibre material which thus often leaves the grinding apparatus in insufficiently desintegrated condition.